1. Field of the Invention
The demands made of modern internal combustion engines, whether they are externally ignited mixture-compressing or air-compressing engines, in terms of exhaust emissions are becoming more and more stringent. For the European IV standard expected for 2005, in air-compressing internal combustion engines, the use of particle filters is being discussed. They are intended to remove particles, emitted by the Diesel engine, from the exhaust gas stream. CO and HC can be combusted for the most part using oxidation-type catalytic converters. The reduction of NOx, which moreover occurs in externally ignited gasoline direct-injected Otto engines as well, proves difficult.
2. Prior Art
In air-compressing internal combustion engines, the pollutants, that is, CO and HC, are for most part combusted with an oxidation-type catalytic converter downstream of the engine. What proves most difficult is the reduction of NOx. Only by adding a reducing agent to the exhaust gas flow can the NOx reduction be achieved in a downstream catalytic converter.
The provision of an oxidation catalytic converter downstream in a Diesel engine causes a marked reduction in the emissions of carbon monoxide and hydrocarbons. Since hydrocarbon emissions contribute to particle emissions, they too can be reduced to a limited extent by the catalytic converter. The use of low-sulfur fuel (≦0.001% sulfur (<10 ppm), as of 2005) assures durable effectiveness of the catalytic converter.
At present, the highest NOx conversion rates are achieved by means of the SCR (selective catalytic reduction) method. In it, a reducing agent is sprayed into the multi-part catalytic converter via a metering system as a function of a performance graph. The reducing agent used most often at present is a urea-water solution. In the hydrolysis part, the actual reducing agent, ammonia, is formed from the solution. In the SCR part, the reduction of NOx then takes place. The catalytic converter system is as a rule completed by an oxidation part, which oxidizes the uncombusted pollutants, that is, CO and HC. Instead of a urea-water solution as a reducing agent, Diesel fuel can also be used as a reducing agent.
In another variant embodiment (CRT system), the pollutants, that is, CO, HC and NOx, are first oxidized in an oxidation catalytic converter, using a continuously regenerating particle filter system. The NO2 formed by the oxidation then bonds in the downstream soot filter with the carbon C in the particles deposited there and combusts it continuously to form CO2. The NO2 is thus further reduced to NO. If a complete NOx reduction is desired, it would have to be performed subsequently on the basis of the SCR method—as described above. This system avoids temperature peaks in the soot filter and thus lengthens its service life. To assure optimal, durably effective reduction, it is necessary to operate the engine with sulfur-free Diesel fuel (≦0.001% sulfur), also known as city Diesel.
In Otto engines currently in development, which operate on that principle of direct gasoline injection, NOx components in the very lean exhaust gas that occur in combustion cannot be reduced by a three-way catalytic converter. By recirculating exhaust gas at a high exhaust gas recirculation rate, a reduction in the NOx component of the exhaust gas of about 70% is achieved. If exhaust gas regulations are to be met, an additional posttreatment of the NOx emissions cannot be avoided. For reducing this component of the exhaust gas, the NOx storage-type catalytic converter offers the greatest potential. In terms of measures taken in the engine to vary the exhaust gas composition, in terms of NOx emissions, it has proven very effective to achieve the lowering in the peak combustion temperature through the exhaust gas that is returned to the combustion chamber again. Since the NOx development increases disproportionately with the combustion temperature, exhaust gas recirculation, as a provision to reduce temperature, is a highly effective method of NOx reduction. By further optimizing the exhaust gas recirculation rates, a reduction in fuel consumption can also be attained. The exhaust gas recirculation can be achieved by way of internal exhaust gas recirculation by means of suitable valve overlap, or by external exhaust gas recirculation through suitably controlled exhaust gas recirculation valves.